Reference signal transmission method for downlink multiple input multiple output system

ABSTRACT

A reference signal transmission method in a downlink MIMO system is disclosed. The downlink MIMO system supports a first UE supporting N transmission antennas among a total of M transmission antennas (where M&gt;N) and a second UE supporting the M transmission antennas. The method includes transmitting, by a base station (BS), subframe-associated information which designates a first subframe in which data for the first UE and the second UE is transmitted and a second subframe in which data only for the second UE can be transmitted within a radio frame having a plurality of subframes, and transmitting the first subframe and the second subframe. Reference signals corresponding to antenna ports ‘0’ to ‘N−1’ of the N antennas are mapped to the first subframe, and reference signals corresponding to antenna ports ‘0’ to ‘M−1’ of the M antennas are mapped to the second subframe.

TECHNICAL FIELD

The present invention relates to a Multiple Input Multiple Output (MIMO)communication system, and more particularly to a method for effectivelyproviding data and a reference signal under an environment in which anantenna is added to a conventional system.

BACKGROUND ART

(1) Definition of MIMO Technology

A conventional MIMO technology will hereinafter be described in detail.

In brief, MIMO technology is an abbreviation for

Multi-Input Multi-Output technology. MIMO technology uses multipletransmission (Tx) antennas and multiple reception (Rx) antennas toimprove the efficiency of Tx/Rx data, whereas a conventional art hasgenerally used one transmission (Tx) antenna and one reception (Rx)antenna. In other words, MIMO technology allows a transmission end orreception end of a wireless communication system to use multipleantennas (hereinafter referred to as a multi-antenna), so that thecapacity or performance can be improved. For convenience of description,the term “MIMO” can also be considered to be a multi-antenna technology.

In more detail, MIMO technology is not dependent on one antenna path toreceive one total message, collects a plurality of data pieces receivedvia several antennas, and completes total data. As a result, MIMOtechnology can increase a data transfer rate within a specific range, orcan increase a system range at a specific data transfer rate. Under thissituation, MIMO technology is a next-generation mobile communicationtechnology capable of being widely applied to mobile communicationterminals or repeaters. MIMO technology can extend the range of datacommunication, so that it can overcome the limited amount oftransmission (Tx) data of mobile communication systems reaching acritical situation.

(2) System Modeling in MIMO

FIG. 1 is a block diagram illustrating a general MIMO communicationsystem.

Referring to FIG. 1, if the number of transmission (Tx) antennasincreases to N_(t), and at the same time the number of reception (Rx)antennas increases to N_(R), a theoretical channel transmission capacityof the MIMO communication system increases in proportion to the numberof antennas, differently from the above-mentioned case in which only atransmitter or receiver uses several antennas, so that a transfer rateand a frequency efficiency can be greatly increased. In this case, thetransfer rate acquired by the increasing channel transmission capacitycan theoretically increase by a predetermined amount that corresponds tomultiplication of a maximum transfer rate (R_(o)) acquired when oneantenna is used and a rate of increase (R_(i)). The rate of increase(R_(i)) can be represented by the following equation 1.

R _(i)=min(N _(T) ,N _(R))   [Equation 1]

For example, provided that a MIMO system uses four transmission (Tx)antennas and four reception (Rx) antennas, the MIMO system cantheoretically acquire a high transfer rate which is four times higherthan that of a one antenna system. After the above-mentioned theoreticalcapacity increase of the MIMO system was demonstrated in the mid-1990s,many developers began to conduct intensive research into a variety oftechnologies which can substantially increase a data transfer rate usingthe theoretical capacity increase. Some of the above technologies havebeen reflected in a variety of wireless communication standards, forexample, a third-generation mobile communication or a next-generationwireless LAN, etc.

A variety of MIMO-associated technologies have been intensivelyresearched by many companies or developers, for example, research intoan information theory associated with a MIMO communication capacitycalculation under various channel environments or multiple accessenvironments, research into a radio frequency (RF) channel measurementand modeling of the MIMO system, and research into a space-time signalprocessing technology. A mathematical modeling of a communication methodfor use in the above-mentioned MIMO system will hereinafter be describedin detail.

As can be seen from FIG. 1, it is assumed that there are N_(T)transmission (Tx) antennas and N_(R) reception (Rx) antennas. In thecase of a transmission (Tx) signal, a maximum number of transmissioninformation pieces is N_(T) under the condition that N_(T) transmission(Tx) antennas are used, so that the transmission (Tx) information can berepresented by a specific vector shown in the following equation 2.

s=└s₁,s₂, . . . ,s_(N) _(T) ┘^(T)   [Equation 2]

In the meantime, individual transmission (Tx) information pieces (s₁,s₂, . . . , s_(NT)) may have different transmission powers. In thiscase, if the individual transmission powers are denoted by (P₁, P₂, . .. , P_(NT)), transmission (Tx) information having an adjustedtransmission power can be represented by a specific vector shown in thefollowing equation 3.

ŝ=[ŝ₁,ŝ₂, . . . ,ŝ_(N) _(T) ]^(T)=[P₁s₁,P₂s₂, . . . ,P_(N) _(T) s_(N)_(T) ]^(T)   [Equation 3]

In Equation 3, ŝ is a transmission vector, and can be represented by thefollowing equation 4 using a diagonal matrix P of a transmission (Tx)power.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

In the meantime, the information vector Ŝ having an adjustedtransmission power is applied to a weight matrix (W), so that N_(T)transmission (Tx) signals (x₁, x₂, . . . , x_(NT)) to be actuallytransmitted are configured. In this case, the weight matrix (W) isadapted to properly distribute transmission (Tx) information toindividual antennas according to transmission channel situations. Theabove-mentioned transmission (Tx) signals (x₁, x₂, . . . , x_(NT)) canbe represented by the following equation 5 using the vector (X).

$\begin{matrix}\begin{matrix}{x = \begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix}} \\{= {\begin{bmatrix}w_{11} & w_{12} & \cdots & w_{1\; N_{T}} \\w_{21} & w_{22} & \cdots & w_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \cdots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \cdots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}}} \\{= {W\hat{s}}} \\{= {WPs}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Next, if N_(R) reception (Rx) antennas are used, reception (Rx) signals(y₁, y₂, . . . , y_(NR)) of individual antennas can be represented by aspecific vector (y) shown in the following equation 6.

y=[y₁,y₂, . . . ,y_(N) _(R) ]^(T)   [Equation 6]

In the meantime, if a channel modeling is executed in the MIMOcommunication system, individual channels can be distinguished from eachother according to transmission/reception (Tx/Rx) antenna indexes. Aspecific channel passing the range from a transmission (Tx) antenna (j)to a reception (Rx) antenna (i) is denoted by h_(ij). In this case, itshould be noted that the index order of the channel h_(ij) is locatedbefore a reception (Rx) antenna index and is located after atransmission (Tx) antenna index.

Several channels are tied up, so that they are displayed in the form ofa vector or matrix. An exemplary vector is as follows. FIG. 2 showschannels from N_(T) transmission (Tx) antennas to a reception (Rx)antenna (i).

Referring to FIG. 2, the channels passing the range from the N_(T)transmission (Tx) antennas to the reception (Rx) antenna (i) can berepresented by the following equation 7.

h_(i) ^(T)=└h_(i1),h_(i2), . . . ,h_(iN) _(T) ┘

If all channels passing the range from the N_(T) transmission (Tx)antennas to N_(R) reception (Rx) antennas are denoted by the matrixshown in Equation 7, the following equation 8 is acquired.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \cdots & h_{1\; N_{T}} \\h_{21} & h_{22} & \cdots & h_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \cdots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \cdots & h_{N_{R}N_{T}}\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

Additive white Gaussian noise (AWGN) is added to an actual channel whichhas passed the channel matrix (H) shown in Equation 8. The AWGN (n₁, n₂,. . . , n_(NR)) added to each of N_(R) reception (Rx) antennas can berepresented by a specific vector shown in the following equation 9.

n=[n₁,n₂, . . . ,n_(N) _(R) ]^(T)   [Equation 9]

A reception signal calculated by the above-mentioned equations can berepresented by the following equation 10.

$\begin{matrix}\begin{matrix}{y = \begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix}} \\{= {{\begin{bmatrix}h_{11} & h_{12} & \cdots & h_{1\; N_{T}} \\h_{21} & h_{22} & \cdots & h_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \cdots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \cdots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}}} \\{= {{Hx} + n}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

The 3^(rd) Generation Partnership Project (3GPP) supports a type 1 radioframe structure applicable to frequency division duplex (FDD), and atype 2 radio frame structure applicable to time division duplex (TDD).

The structure of a type 1 radio frame is shown in FIG. 3. The type 1radio frame includes ten subframes, each of which consists of two slots.

The structure of a type 2 radio frame is shown in FIG. 4. The type 2radio frame includes two half-frames, each of which is made up of fivesubframes, a downlink pilot time slot (DwPTS), a guard period (GP), andan uplink pilot time slot (UpPTS), in which one subframe consists of twoslots. That is, one subframe is composed of two slots irrespective ofthe radio frame type. DwPTS is used to perform an initial cell search,synchronization, or channel estimation. UpPTS is used to perform channelestimation of a base station and uplink transmission synchronization ofa user equipment (UE). The guard interval (GP) is located between anuplink and a downlink so as to remove an interference generated in theuplink due to a multi-path delay of a downlink signal. That is, onesubframe is composed of two slots irrespective of the radio frame type.

FIG. 5 is a slot structure of a long term evolution (LTE) downlink. Asshown in FIG. 5, a signal transmitted from each slot can be described bya resource grid including N_(RB) ^(DL) N_(SC) ^(RB) subcarriers andN_(symb) ^(DL) orthogonal frequency division multiplexing (OFDM)symbols. In this case, N_(RB) ^(DL) represents the number of resourceblocks (RBs) in a downlink, N_(SC) ^(RB) represents the number ofsubcarriers constituting one RB, and N_(symb) ^(DL) represents thenumber of OFDM symbols in one downlink slot.

FIG. 6 is a slot structure of a long term evolution (LTE) uplink. Asshown in FIG. 6, a signal transmitted from each slot can be described bya resource grid including N_(RB) ^(UL)N_(SC) ^(RB) subcarriers andN_(symb) ^(UL) OFDM symbols. In this case, N_(RB) ^(UL) represents thenumber of resource blocks (RBs) in an uplink, N_(SC) ^(RB) representsthe number of subcarriers constituting one RB, and N_(symb) ^(UL)represents the number of OFDM symbols in one uplink slot.

A resource element (RE) is a resource unit defined by an index (a, b)within the uplink slot and the downlink slot, and represents onesubcarrier and one OFDM symbol. In this case, a is an index on afrequency axis, and b is an index on a time axis.

4) Definition of Reference Signal

When a mobile communication system transmits a packet, this transmissionpacket is transmitted over a radio frequency (RF) channel. As a result,an unexpected distortion may occur in a transmission (Tx) signal. Inorder to correctly receive the distorted signal described above at areception end, channel information must be recognized, and thedistortion of the transmission (Tx) signal must be corrected by anamount of the channel information. In order to recognize channelinformation, signals known to both a transmission end and a receptionend are transmitted, the degree of distortion of the known signals isdetected when the known signals are received over a channel, and finallychannel information is recognized on the basis of the detecteddistortion.

Here, the above signals known to both the transmission end and thereception end are referred to as pilot signals or reference signals.

In recent times, most mobile communication systems use a method forimproving Tx/Rx data efficiency using multiple transmission (Tx)antennas and multiple reception (Rx) antennas to transmit a packet,instead of a conventional method of using one transmission (Tx) antennaand one reception (Rx) antenna to transmit a packet. When a transmissionend or a reception end of a mobile communication system transmits orreceives data using multiple antennas so as to increase capacity orimprove a performance or throughput, additional reference signals arepresent in individual transmission (Tx) antennas, respectively. Signalreception can be correctly carried out under the condition that achannel condition between each Tx antenna and each Rx antenna isrecognized.

Provided that M (where M>N) transmission (Tx) antennas can be added to aconventional system including N antennas, a user equipment (hereinafterreferred to as a UE) capable of recognizing up to N transmission (Tx)antennas and another UE capable of recognizing up to M transmission (Tx)antennas exist at the same time.

In this case, not only reference signals for supporting N antennas butalso M−N additional reference signals must be transmitted. Here, thereis a need to effectively transmit data and reference signals under anenvironment in which a new UE for recognizing M antennas is additionallyused without performing additional signaling with an old UE forrecognizing only N antennas.

DISCLOSURE Technical Problem

Accordingly, the present invention is directed to a method fortransmitting a reference signal in a downlink MIMO system thatsubstantially obviates one or more problems due to limitations anddisadvantages of the related art.

An object of the present invention is to provide a method foreffectively transmitting data and a reference signal under anenvironment in which a UE capable of supporting N transmission (Tx)antennas and another UE capable of supporting M reception (Rx) antennassimultaneously exist.

Technical Solution

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, amethod for transmitting a reference signal for channel measurement in adownlink Multiple Input Multiple Output (MIMO) system which supports afirst user equipment (UE) supporting N transmission antennas among atotal of M transmission antennas (where M>N) and a second user equipment(UE) supporting the M transmission antennas includes: transmitting, by abase station (BS), subframe-associated information which designates afirst subframe in which data for the first UE and the second UE istransmitted and a second subframe in which data only for the second UEcan be transmitted within a radio frame having a plurality of subframes;and transmitting the first subframe and the second subframe, whereinreference signals corresponding to antenna ports ‘0’ to ‘N−1’ of the Nantennas are mapped to the first subframe, and reference signalscorresponding to antenna ports ‘0’ to ‘M−1’ of the M antennas are mappedto the second subframe.

In another aspect of the present invention, a base station (BS) for usein a downlink Multiple Input Multiple Output (MIMO) system whichsupports a first user equipment (UE) supporting N transmission antennasamong a total of M transmission antennas (where M>N) and a second userequipment (UE) supporting the M transmission antennas includes: aprocessing unit for generating subframe-associated information whichdesignates a first subframe in which data for the first UE and thesecond UE can be transmitted and a second subframe in which data onlyfor the second UE can be transmitted within a radio frame having aplurality of subframes; and a transmitter for transmitting thesubframe-associated information, the first subframe, and the secondsubframe, wherein reference signals corresponding to antenna ports ‘0’to ‘N−1’ of the N antennas are mapped to the first subframe, andreference signals corresponding to antenna ports ‘0’ to ‘M−1’ of the Mantennas are mapped to the second subframe.

Positions of reference signals corresponding to antenna ports ‘0’ to‘N−1’ among the M antennas in the second subframe may be equal to thoseof the reference signals corresponding to the antenna ports ‘0’ to ‘N−1’among the N antennas in the first subframe.

The second subframe may include a plurality of orthogonal frequencydivision multiplexing (OFDM) symbols, a first portion of the OFDMsymbols is for transmitting control information, a second portion of theOFDM symbols is for transmitting data, and reference signalscorresponding to antenna ports ‘N’ to ‘M−1’ among the M antennas may bemapped to the second portion.

In the subframe-associated information, the second subframe may bedesignated as a multicast broadcast single frequency network (MBSFN)subframe.

In a further aspect of the present invention, a method for transmittinga reference signal for channel measurement in a downlink Multiple InputMultiple Output (MIMO) system includes: transmitting, by a base station(BS), subframe-associated information which designates a first subframein which N reference signals are transmitted and a second subframe Mreference signals (where M>N) within a radio frame; and transmitting thefirst subframe and the second subframe, wherein reference signalscorresponding to antenna ports ‘0’ to ‘N−1’ among the N antennas aremapped to the first subframe, and reference signals corresponding toantenna ports ‘0’ to ‘M−1’ among the M antennas are mapped to the secondsubframe.

In a further aspect of the present invention, a base station (BS) foruse in a downlink Multiple Input Multiple Output (MIMO) system includes:a processing unit for generating subframe-associated information whichdesignates a first subframe in which N reference signals are transmittedand a second subframe M reference signals (where M>N) within a radioframe; and a transmitter for transmitting the subframe-associatedinformation, the first subframe, and the second subframe, whereinreference signals corresponding to antenna ports ‘0’ to ‘N−1’ of the Nantennas are mapped to the first subframe, and reference signalscorresponding to antenna ports ‘0’ to ‘M−1’ of the M antennas are mappedto the second subframe.

Positions of reference signals corresponding to antenna ports ‘N’ to‘M−1’ among the M antennas in the second subframe may be equal to thoseof the reference signals corresponding to the antenna ports ‘0’ to ‘N−1’of the N antennas in the first subframe.

Advantageous Effects

According to embodiments of the present invention, it is possible toeffectively transmit data and a reference signal under an environment inwhich one UE for supporting N transmission (Tx) antennas and another UEfor supporting M transmission (Tx) antennas simultaneously exist in adownlink MIMO system.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 is a block diagram illustrating a general MIMO communicationsystem.

FIG. 2 shows channels from N_(T) transmission (Tx) antennas to areception (Rx) antenna (i).

FIG. 3 is a structure of a type 1 radio frame.

FIG. 4 is a structure of a type 2 radio frame.

FIG. 5 is a slot structure of an LTE downlink.

FIG. 6 is a slot structure of an LTE uplink.

FIG. 7 is a structure of a radio frame according to an exemplaryembodiment of the present invention.

FIG. 8 is a structure of a subframe for supporting 8 antennas accordingto an exemplary embodiment of the present invention.

FIG. 9 is a structure of a subframe for supporting 8 antennas accordingto an exemplary embodiment of the present invention.

FIG. 10 is a structure of a subframe for supporting 8 antennas accordingto an exemplary embodiment of the present invention.

FIG. 11 is a structure of a subframe for supporting 8 antennas accordingto an exemplary embodiment of the present invention.

FIG. 12 is a structure of a subframe for supporting 8 antennas accordingto an exemplary embodiment of the present invention.

FIG. 13 is a structure of a subframe for discriminating between oneregion for supporting N antennas and another region for supporting M(M>N) antennas on the basis of a frequency axis according to anexemplary embodiment of the present invention.

FIG. 14 is a structure of a radio frame that distinguishes one regionfor supporting N antennas from another region for supporting M (M>N)antennas on a frequency axis according to an exemplary embodiment of thepresent invention.

FIG. 15 is a block diagram of a device, which is applicable to a userequipment (UE) and a base station (BS) and is able to implementembodiments of the present invention.

BEST MODE

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description, which will be given below withreference to the accompanying drawings, is intended to explain exemplaryembodiments of the present invention, rather than to show the onlyembodiments that can be implemented according to the present invention.

The following detailed description includes specific details in order toprovide a thorough understanding of the present invention. However, itwill be apparent to those skilled in the art that the present inventionmay be practiced without such specific details. For example, thefollowing description will be given centering on specific terms, but thepresent invention is not limited thereto and any other terms may be usedto represent the same meanings. Also, wherever possible, the samereference numbers will be used throughout the drawings to refer to thesame or like parts.

In the case where M−N reference signals are additionally transmitted tosupport M (where M>N) antennas under an environment in which N referencesignals are transmitted to support N transmission (Tx) antennas(hereinafter referred to simply as ‘antennas’), it is difficult todefine a new additional signaling for informing the user equipment (UE),which recognizes only N antennas operating in a conventional system, ofthe above-mentioned case representing additional transmission of the M−Nreference signals. Therefore, the UE capable of recognizing only Nantennas is unable to recognize transmission of M reference signals, sothat an unexpected problem occurs in transmission and reception of data.

In order to solve the above-mentioned problem, the present inventionproposes a method for dividing a radio frame into one transmissioninterval for supporting only N antennas and another transmissioninterval for supporting up to M antennas in addition to the N antennas.

Because the UE capable of recognizing only N antennas has difficulty innewly defining additional signaling in a transmission interval, thepresent invention provides a method for enabling a base station (BS) torestrict UEs capable of recognizing only N antennas so as to transmitdata to only the transmission interval supporting only the N antennas.

In addition, the present invention provides a method for signalinginformation about a transmission interval supporting N antennas and atransmission interval supporting M antennas to a UE capable ofrecognizing up to M antennas. Thus, the UE capable of recognizing up toM antennas can transmit data in any one of the transmission intervals(the transmission interval supporting N antennas and the transmissioninterval supporting M antennas).

FIG. 7 is a structure of a radio frame according to an exemplaryembodiment of the present invention. In FIG. 7, a radio frame may be afrequency divisional duplex (FDD) radio frame. As can be seen from FIG.7, a transmission interval of the radio frame is divided into aplurality of subframes, each of which supports a maximum of N antennas,and a plurality of other subframes, each of which supports a maximum ofM antennas. An overall data transmission interval defined by a radioframe includes 10 transmission intervals, each of which is defined by asubframe. FIG. 7 is an example of a method for distinguishing onesubframe for supporting N antennas from another subframe for supportingM antennas. The subframe for supporting N antennas and the othersubframe for supporting M antennas may be dynamically or semi-staticallychanged.

In this case, a UE capable of recognizing only N antennas is unable torecognize the presence of M−N additional reference signals in a subframecapable of supporting M antennas, so that there is a need to performscheduling restriction so as to prevent the UE recognizing only Nantennas from receiving data in the subframe. The other UE recognizing Mantennas is able to receive data in a subframe supporting N antennas. Inthis case, because information of one transmission interval forsupporting N antennas and information of another transmission intervalfor supporting M antennas are signaled to the UE recognizing M antennas,although the UE recognizing M antennas receives data in the transmissioninterval supporting N antennas, channel information of M−N additionalantennas can be recognized from a neighboring subframe supporting Mantennas.

In this case, in order to support M antennas in a specific subframe, thepresent invention proposes a method for transmitting additional M−Nreference signals in addition to reference signals for supporting Nantennas.

For convenience of description and better understanding of the presentinvention, a 3rd Generation Partnership Project (3GPP) Long TermEvolution (LTE) system may be exemplarily used as a first system forsupporting the UE recognizing only N antennas, and a LTE-A system may beexemplarily used as a second system for supporting the other UErecognizing M antennas, and a detailed description thereof will be givenbelow with reference to the accompanying drawings.

FIG. 8 is a structure of a subframe for supporting 8 antennas accordingto an exemplary embodiment of the present invention. As shown in FIG. 8,four reference signals (R1 to R4) for respectively supporting fourantennas (Ant1 to Ant4) are transmitted in a subframe of the LTE system,and four reference signals (R5 to R8) in addition to conventionalreference signals (R1 to R4) can be transmitted in subframes of theLTE-A system so as to support eight antennas (Ant1 to Ant8).

In the case where data for a UE capable of recognizing only fourantennas is transmitted in a subframe in which eight reference signals(R1 to R8) are transmitted, the UE is unable to recognize transmissionof other reference signals (R5 to R8), so that the UE regards thereference signals (R5 to R8) as data transmitted to the UE. As a result,the UE demodulates the reference signals (R5 to R8) and decodes thedemodulated reference signals. In this case, transmission performancemay be deteriorated due to reception of incorrect information.Accordingly, data for the UE capable of recognizing only four antennasshould not be allocated to a subframe in which eight reference signals(R1 to R8) are transmitted.

In contrast, in case of another UE capable of recognizing eightantennas, although data is transmitted in a subframe in which only fourreference signals (R1 to R4) are transmitted through four or moreantennas, the UE can obtain channel information of the remaining fourantennas (Ant5 to Ant8) in addition to other channel informationrecognized on the basis of the four reference signals (R1 to R4) from aneighboring subframe supporting 8 antennas.

In this case, a first to a third OFDM symbols (OFDM symbol index 0 toOFDM symbol index 2) of each subfrarme are used to transmit a channel(i.e., a control channel) containing control information. It isprescribed that a maximum of four antennas can be used within aninterval in which the above control channel is used, so that all UEs canreceive the control channel irrespective of the number of recognizableantennas.

If the number of symbols transmitting the control channel is deficient,a fourth OFDM symbol may also be used to transmit the control channel.Here, when transmitting reference signals using the structure shown inFIG. 8, UEs supporting four antennas are unable to recognize tworeference signals R5 and R6 contained in the fourth OFDM symbol. Tosolve the above problem, in case that the first to the forth OFDMsymbols are used for the control channel, new additional referencesignals (R5 to R8) may be arranged from a fifth OFDM symbols.

FIG. 9 is a structure of a subframe for supporting 8 antennas accordingto an exemplary embodiment of the present invention. As can be seen fromFIG. 9, only four reference signals (R1 to R4) are transmitted in thefirst four OFDM symbols in a subframe in which eight reference signals(R1 to R8) are transmitted, so that all UEs can receive a controlchannel irrespective of the number of recognizable antennas. Accordingto the above-mentioned structure for applying additional referencesignals to N reference signals supporting N antennas, a performance orthroughput of a UE recognizing only N antennas may be unexpectedlydeteriorated as previously described above. Accordingly, it isundesirable that data for the UE recognizing only N antennas betransmitted in the subframe supporting M antennas, but the UErecognizing N antennas can use channel information in the subframesupporting M antennas.

The subframe supporting M antennas includes reference signals forsupporting N antennas, so that it is possible to perform interpolationor averaging of channel information between subframes supporting Nantennas. FIGS. 8 and 9 exemplarily show a method for transmittingadditional four reference signals (R5 to R8) so as to support a maximumof eight reference signals in a conventional subframe structure in whicha maximum of four reference signals (R1 to R4) are transmitted. In thiscase, the number of used reference signals is disclosed only forillustrative purposes, and embodiments of the present invention can alsobe applied to other variables N and M (where M>N). In addition, in caseof additionally-transmitted reference signals differently fromconventional reference signals, embodiments of the present inventionpropose a CDM-based transmission method between theadditionally-transmitted reference signals. In case of using theCDM-based transmission method, many more reference signals can betransmitted using the same resources as those of the conventionalreference signals, and resource consumption can be reduced due to thetransmission of the same reference signals, so that the CDM-basedtransmission method can use resources more effectively than theconventional method.

However, in the case where additional reference signals are transmittedin addition to N conventional reference signals in the subframesupporting M antennas, much more resources are needed to transmit theadditional reference signals, so that resources capable of being used totransmit data are relatively reduced. In order to solve the aboveproblem, embodiments of the present invention propose a new method.According to this new method, all M reference signals supporting Mantennas are transmitted in the subframe supporting M antennas. However,the M transmission reference signals are not transmitted using aconventional structure transmitting N conventional reference signals,but are transmitted using a new structure.

In case of transmitting M reference signals using the above newstructure without being added to such a conventional structure,embodiments of the present invention can more effectively use time andfrequency resources needed for transmitting reference signals. Inaddition, the present invention proposes a method for transmitting onlyM−N additional reference signals, instead of N conventional referencesignals, in the subframe supporting M antennas. According to thisproposed method, under the condition that there is no need to use theconventional structure for transmitting N reference signals without anychange, an interval between reference signals may be adjusted onfrequency and time axes of corresponding reference signals inconsideration of not only the number of transmitted reference signalsbut also channel conditions to which additional reference signals areapplied, and the total number of reference signals may also be adjusted.

Further, the present invention proposes another method. According tothis method, the structure for use in the transmission of N referencesignals is maintained without any change, reference signals ofcorresponding positions are replaced with new additional referencesignals, and then the new additional reference signals can betransmitted.

FIG. 10 is a structure of a subframe for supporting 8 antennas accordingto an exemplary embodiment of the present invention. As shown in FIG.10, in a subframe supporting eight antennas, four new reference signals(R5 to R8) are transmitted on a specific position on which fourconventional reference signals (R1 to R4) are transmitted. In this case,additional resources for transmitting additional reference signals (R5to R8) need not be used for the above-mentioned transmission, so thatresources can be effectively used. In this way, when using a method fortransmitting additional reference signals in a specific subframe so asto support M antennas, specific information is signaled to a UE capableof recognizing the M antennas, where the specific information indicateswhich subframe is a subframe in which additional reference signals forsupporting the M antennas are transmitted. Because the specificinformation is signaled to the UE capable of recognizing the M antennas,the UE can obtain channel information from any subframe based on thespecific information. Thus, the system can be operated stably.

However, the UE capable of recognizing N conventional antennas is unableto recognize the above information. Accordingly, when channelinformation between subframes is interpolated or averaged, the UE mayuse incorrect channel information of the subframe in which additionalreference signals are transmitted, instead of using information ofanother subframe transmitting N conventional reference signals,resulting in the occurrence of unexpected problems.

In order to solve the above-mentioned problems, the present inventionproposes a signaling method for preventing a UE capable of recognizingonly N antennas from receiving a subframe supporting M antennas. Avariety of signaling methods may be used in the present invention. Forexample, the following four signaling methods (1)˜(4) may be used inembodiments of the present invention, and a detailed description thereofwill be given below.

1) First Signaling Method

The first signaling method is used to distinguish one subframe in whichN reference signals are used, from another subframe in which N referencesignals and additional reference signals are used or other referencesignals for additional antenna ports are used.

2) Second Signaling Method

The second signaling method allows a UE capable of recognizing only Nreference signals to use reference signals of a subframe supporting onlyN reference signals to which data is allocated, without using referencesignals of a neighboring subframe supporting M antennas.

3) Third Signaling Method

The third signaling method allows a UE capable of recognizing only Nreference signals to use channel information of a specific subframesupporting only the N reference signals, instead of using channelinformation of another subframe supporting M antennas, even though thedata is not allocated to the specific subframe.

4) Fourth Signaling Method

Any one of known signaling methods may be exemplarily used for the UErecognizing only N antennas. For example, in the 3GPP LTE system, a basestation (BS) may signal to UEs capable of recognizing a maximum of fourantennas that specific subframe is a Multicast Broadcast SingleFrequency Network (MBSFN) subframe.

The fourth signaling method (4) among the above-mentioned signalingmethods will hereinafter be described in detail. If the 3GPP LTE systemperforms signaling with a corresponding UE so as to indicate the MBSFNsubframe, this UE does not use channel information of the MBSFN subframebecause it does not read a data part of the MBSFN subframe, and useseither only channel information of a subframe which the UE receives oronly channel information of another subframe supporting four antennas,so that no problems occur in the system.

In other words, in order to control a UE recognizing only four antennasto recognize a subframe supporting 8 antennas as a MBSFN subframe, thebase station (BS) can signal to the UE that the subframe supportingeight antennas is the MBSFN subframe.

Although the subframe supporting eight antennas is not actual MBSFNsubframe, the UE recognizing only four antennas can recognize thesubframe supporting eight antennas as the MBSFN subframe according tothe signaling result of the fourth signaling method. In the meantime, ifnecessary, another additional signaling process may also be required forthe UE recognizing eight antennas so as to recognize the subframesupporting eight antennas.

However, as previously mentioned above, the first one to three OFDMsymbols of a subframe (or the first two to four OFDM symbols of asubframe in some cases) may be transmitted to transmit a channel (i.e.,control channel) containing control information. Here, a transmissioninterval of the control channel may be defined as a specifictransmission interval in which a maximum of four antennas can be used sothat all UEs can receive the control channel regardless of the number ofrecognizable antennas. In case of transmitting reference signals asshown in FIG. 10, the UE recognizing four conventional antennas isunable to recognize new reference signals (R5 to R8), so that anunexpected problem occurs in reception of the control channel.

Accordingly, in order to solve the above-mentioned problem, embodimentsof the present invention propose another method. According to thismethod, four conventional reference signals (R1 to R4) are transmittedin an interval through which the control channel is transmitted, andoriginal reference signals (R1 to R4) are replaced with new referencesignals (R5 to R8). The new reference signals (R5 to R8) are transmittedin an interval in which data is transmitted.

FIG. 11 is a structure of a subframe for supporting eight antennasaccording to an exemplary embodiment of the present invention.

As shown in FIG. 11, conventional reference signals (R1 to R4) may betransmitted in a first region (i.e., first and second OFDM symbolillustrated in FIG. 11) in which a control channel is transmitted, andthe conventional reference signals (R1 to R4) are replaced with newreference signals (R5 to R8) in a second region in which data istransmitted, so that the resultant subframe composed of the conventionalreference signals (R1 to R4) and the new reference signals (R5 to R8)may be transmitted. In this case, the present invention may use aspecific signaling method for preventing a UE recognizing only Nantennas from reading the second region.

In this way, if the above-mentioned signaling method prevents the UEcapable of recognizing only the N antennas from reading the secondregion, there is no need to transmit the new reference signals to thesame positions as those of the N reference signals, and the newreference signals can be transmitted to other positions where channelestimation capability may greatly increase.

FIG. 12 is a structure of a subframe for supporting antennas accordingto an exemplary embodiment of the present invention. As shown in FIG.12, transmission positions of new reference signals (R5 to R8) arechanged to new positions instead of transmission positions ofconventional reference signals (R1 to R4). In this way, a channelestimation performance can be improved by changing the positions of thereference signals. In order to support a maximum of 8 antennas in thesubframe structure in which a maximum of four conventional referencesignals are transmitted, FIGS. 10 to 12 exemplarily show methods forreplacing four conventional reference signals (R1 to R4) with four newreference signals (R5 to R8) and then transmitting the replaced result.In this case, the number of used reference signals is disclosed only forillustrative purposes, and embodiments of the present invention may setthe number of such reference signals to N and M (where M>N).

In the case of using new reference signals newly transmitted in thesubframe supporting M antennas according to embodiments of the presentinvention, a Code Division Multiplexing (CDM)-based transmission methodmay be used between the newly-transmitted reference signals in adifferent way from conventional reference signals. In case of using theCDM-based transmission method, many more reference signals can betransmitted using the same resources as those of the conventionalreference signals, an amount of used resources can be reduced whentransmitting the same reference signals, so that the CDM-basedtransmission method can use resources more effectively than theconventional method.

Further, although the above-mentioned proposed methods have beendisclosed to restrict UE allocation by classifying subframes into onesubframe capable of supporting only N antennas and another subframecapable of supporting up to M antennas (where M>N) on a time axis,embodiments of the present invention propose a method for discriminatingbetween reference signals on a frequency axis in a data region allocatedto a UE recognizing a maximum of M antennas.

FIG. 13 is a structure of a subframe for discriminating between oneregion for supporting N antennas and another region for supporting M(M>N) antennas on the basis of a frequency axis according to anexemplary embodiment of the present invention. As shown in FIG. 13, acontrol region may support only N antennas so as to allow all UEs toreceive data irrespective of the number of recognizable antennas. A dataregion is divided into one data region allocated to one UE recognizingonly N antennas and another data region allocated to another UErecognizing a maximum of M antennas, so that it can support differentreference signals. In this case, the data region allocated to the UErecognizing only N antennas and the other data region allocated to theother UE recognizing a maximum of M antennas may be established invarious formats.

In this case, in order to implement reference signals transmitted in thedata region allocated to the UE recognizing a maximum of M antennas, theabove described method that additional reference signals are added tothe conventional N reference signals can be applied to the data regionor the above described method that the conventional N reference signalsare replaced with the additional reference signals can be applied to thedata region.

As described above, in the case where the number of supportable antennasis classified only in the data region allocated to the UE capable ofrecognizing M antennas, and then reference signals are transmitted tothis data region, resource allocation between UEs can be moreeffectively carried out.

FIG. 14 is a structure of a radio frame that distinguishes one regionfor supporting N antennas from another region for supporting M (M>N)antennas on a frequency axis according to an exemplary embodiment of thepresent invention. Although one subframe has been exemplarily used inFIG. 13, as shown in FIG. 14, one radio frame may be divided into a dataregion allocated to one UE recognizing only N antennas and another dataregion allocated to another UE recognizing a maximum of M antennas inthe direction of a frequency axis, and then the above-mentioned methodsmay be applied to such data region as necessary.

By the above-mentioned methods, a UE having received reference signalsfrom a base station (BS) can generate channel information using thereceived reference signals, and can feed back the generated channelinformation to the base station (BS).

FIG. 15 is a block diagram of a device, which is applicable to a userequipment (UE) or a base station (BS) and is able to implementembodiments of the present invention. Referring to FIG. 15, a device 150includes a processing unit 151, a memory unit 152, a Radio Frequency(RF) unit 153, a display unit 154, and a user interface unit 155. Theprocessing unit 151 handles physical interface protocol layers. Theprocessing unit 151 provides a control plane and a user plane. Theprocessing unit 151 may perform functions of each layer. The memory unit152, which is electrically connected to the processing unit 151, storesan operating system, application programs, and general files. If thedevice 150 is a UE, the display unit 154 may display various pieces ofinformation and be configured with a Liquid Crystal Display (LCD), anOrganic Light Emitting Diode (OLED), etc. which are known in the art.The user interface unit 155 may be configured to be combined with aknown user interface such as a keypad, a touch screen, or the like. TheRF unit 153, which is electrically connected to the processing unit 151,transmits and receives radio signals.

The exemplary embodiments described hereinabove are combinations ofelements and features of the present invention. The elements or featuresmay be considered selective unless otherwise mentioned. Each element orfeature may be practiced without being combined with other elements orfeatures. Further, the embodiments of the present invention may beconstructed by combining parts of the elements and/or features.Operation orders described in the embodiments of the present inventionmay be rearranged. Some constructions or characteristics of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions or characteristics of anotherembodiment. It is apparent that the present invention may be embodied bya combination of claims which do not have an explicit cited relation inthe appended claims or may include new claims by amendment afterapplication.

The embodiments of the present invention may be achieved by variousmeans, for example, hardware, firmware, software, or a combinationthereof. In a hardware configuration, the embodiments of the presentinvention may be implemented by one or more application specificintegrated circuits (ASICs), digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), processors, controllers,microcontrollers, microprocessors, etc.

In a firmware or software configuration, the embodiments of the presentinvention may be achieved by a module, a procedure, a function, etc.performing the above-described functions or operations. Software codemay be stored in a memory unit and driven by a processor. The memoryunit is located at the interior or exterior of the processor and maytransmit data to and receive data from the processor via various knownmeans.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Therefore,the above-mentioned detailed description must be considered for onlyillustrative purposes instead of restrictive purposes. The scope of thepresent invention must be decided by a rational analysis of claims, andall modifications within equivalent ranges of the present invention arecontained in the scope of the present invention. It is apparent that thepresent invention may be embodied by a combination of claims which donot have an explicit cited relation in the appended claims or mayinclude new claims by amendment after application.

[Mode for Invention]

Various embodiments have been described in the best mode for carryingout the invention.

INDUSTRIAL APPLICABILITY

As apparent from the above description, embodiments of the presentinvention are applicable to a user equipment (UE), a base station (BS),or other devices of a wireless mobile communication system.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A method for transmitting a reference signal for channel measurementin a downlink Multiple Input Multiple Output (MIMO) system whichsupports a first user equipment (UE) supporting N transmission antennasamong a total of M transmission antennas (where M>N) and a second userequipment (UE) supporting the M transmission antennas, the methodcomprising: transmitting, by a base station (BS), subframe-associatedinformation which designates a first subframe in which data for thefirst UE and the second UE is transmitted and a second subframe in whichdata only for the second UE is transmitted within a radio frame having aplurality of subframes; and transmitting the first subframe and thesecond subframe, wherein reference signals corresponding to antennaports ‘0’ to ‘N−1’ of the N antennas are mapped to the first subframe,and reference signals corresponding to antenna ports ‘0’ to ‘M−1’ of theM antennas are mapped to the second subframe.
 2. The method according toclaim 1, wherein positions of reference signals corresponding to antennaports ‘0’ to ‘N−1’ among the M antennas in the second subframe are equalto those of the reference signals corresponding to the antenna ports ‘0’to ‘N−1’ of the N antennas in first subframe.
 3. The method according toclaim 1, wherein the second subframe includes a plurality of orthogonalfrequency division multiplexing (OFDM) symbols, a first portion of theOFDM symbols is for transmitting control information,a second portion ofthe OFDM symbols is for transmitting data, and reference signalscorresponding to antenna ports ‘N’ to ‘M−1’ among the M antennas aremapped to the second portion.
 4. The method according to claim 3,wherein, in the subframe-associated information, the second subframe isdesignated as a multicast broadcast single frequency network (MBSFN)subframe.
 5. A method for transmitting a reference signal for channelmeasurement in a downlink Multiple Input Multiple Output (MIMO) system,the method comprising: transmitting, by a base station (BS),subframe-associated information which designates a first subframe inwhich N reference signals are transmitted and a second subframe Mreference signals (where M>N) within a radio frame; and transmitting thefirst subframe and the second subframe, wherein reference signalscorresponding to antenna ports ‘0’ to ‘N−1’ among the N antennas aremapped to the first subframe, and reference signals corresponding toantenna ports ‘0’ to ‘M−1’ among the M antennas are mapped to the secondsubframe.
 6. The method according to claim 5, wherein positions ofreference signals corresponding to antenna ports ‘N’ to ‘M−1’ in thesecond subframe are equal to those of the reference signalscorresponding to the antenna ports ‘0’ to ‘N−1’ of the N antennas in thefirst subframe.
 7. A method for transmitting a reference signal forchannel measurement in a downlink Multiple Input Multiple Output (MIMO)system which supports a first user equipment (UE) supporting Ntransmission antennas among a total of M transmission antennas (whereM>N) and a second user equipment (UE) supporting the M transmissionantennas, the method comprising: transmitting, by a base station (BS),region-associated information which discriminates, on a frequency axiswithin a radio frame, between a first region in which data for the firstUE and the second UE is transmitted and a second region in which dataonly for the second UE is transmitted; and transmitting a subframecontained in the radio frame to the first UE and the second UE, whereinreference signals corresponding to antenna ports ‘0’ to ‘N−1’ of the Nantennas among the M antennas are mapped to the first region, andreference signals corresponding to antenna ports ‘0’ to ‘M−1’ of the Mantennas are mapped to the second region.
 8. A base station (BS) for usein a downlink Multiple Input Multiple Output (MIMO) system whichsupports a first user equipment (UE) supporting N transmission antennasamong a total of M transmission antennas (where M>N) and a second userequipment (UE) supporting the M transmission antennas, the base station(BS) comprising: a processing unit for generating subframe-associatedinformation which designates a first subframe in which data for thefirst UE and the second UE can be transmitted and a second subframe inwhich data only for the second UE can be transmitted within a radioframe having a plurality of subframes; and a transmitter fortransmitting the subframe-associated information, the first subframe,and the second subframe, wherein reference signals corresponding toantenna ports ‘0’ to ‘N−1’ of the N antennas are mapped to the firstsubframe, and reference signals corresponding to antenna ports ‘0’ to‘M−1’ of the M antennas are mapped to the second subframe.
 9. The basestation (BS) according to claim 8, wherein positions of referencesignals corresponding to antenna ports ‘0’ to ‘N−1’ among the M antennasin the second subframe are equal to those of the reference signalscorresponding to the antenna ports ‘0’ to ‘N−1’ among the N antennas inthe first subframe.
 10. The base station (BS) according to claim 8,wherein the second subframe includes a plurality of orthogonal frequencydivision multiplexing (OFDM) symbols, a first portion of the OFDMsymbols is for transmitting control information, second portion of theOFDM symbols is for transmitting data, and reference signalscorresponding to antenna ports ‘N’ to ‘M−1’ among the M antennas aremapped to the second portion.
 11. The base station (BS) according toclaim 10, wherein, in the subframe-associated information, the secondsubframe is designated as a multicast broadcast single frequency network(MBSFN) subframe.
 12. A base station (BS) for use in a downlink MultipleInput Multiple Output (MIMO) system, the base station (BS) comprising: aprocessing unit for generating subframe-associated information whichdesignates a first subframe in which N reference signals are transmittedand a second subframe in which M reference signals (where M>N) aretransmitted, within a radio frame; and a transmitter for transmittingthe subframe-associated information, the first subframe, and the secondsubframe, wherein reference signals corresponding to antenna ports ‘0’to ‘N−1’ of the N antennas are mapped to the first subframe, andreference signals corresponding to antenna ports ‘0’ to ‘M−1’ of the Mantennas are mapped to the second subframe.
 13. The base station (BS)according to claim 12, wherein positions of reference signalscorresponding to antenna ports ‘N’ to ‘M−1’ among the M antennas in thesecond subframe are equal to those of the reference signalscorresponding to the antenna ports ‘0’ to ‘N−1’ among the N antennas inthe first subframe.
 14. A base station (BS) for use in a downlinkMultiple Input Multiple Output (MIMO) system which supports a first userequipment (UE) supporting N transmission antennas among a total of Mtransmission antennas (where M>N) and a second user equipment (UE)supporting the M transmission antennas, the base station (BS)comprising: a processing unit for generating, region-associatedinformation, which discriminates between a first region in which datafor the first UE and the second UE is transmitted and a second region inwhich data only for the second UE is transmitted on a frequency axiswithin a radio frame having a plurality of subframes; and a transmitterfor transmitting the region-associated information, wherein referencesignals corresponding to antenna ports ‘0’ to ‘N−1’ of the N antennasamong the M antennas are mapped to the first region, and referencesignals corresponding to antenna ports ‘0’ to ‘M−1’ of the M antennasare mapped to the second region.